Dye-sensitized photovoltaic cells were developed by Graetzel et al. in Switzerland. They have advantages, such as low cost, and are attracting attention as new types of photoelectric conversion cells (see, for example, Japanese Patent No. 2664194; Japanese Unexamined Patent Application, First Publication No. 2001-160427, and Michael Graetzel, Nature, United Kingdom, 1991, vol. 353, p. 737.
Dye-sensitized photovoltaic cells generally include a transparent conductive electrode substrate, a working electrode that has a porous film made of oxide semiconductor fine particles (nanoparticles), such as titanium dioxide, and is sensitized with a photo-sensitizing dye on the electrode substrate, a counter electrode provided opposing the working electrode, and an electrolyte containing an oxidation-reduction pair filled between the working electrode and the counter electrode. Such a dye-sensitized photovoltaic cell functions as a photoelectric conversion element that converts light energy into electricity when the oxide semiconductor fine particles are sensitized by the photo-sensitizing dye that absorbs incident light, such as sunlight, thereby generating an electromotive force between the working electrode and the counter electrode.
As the electrolyte, an electrolyte solution is typically used in which an oxidation-reduction pair, such as I−/I3−, is dissolved in an organic solvent, such as acetonitrile. Other well-known electrolytes include one using a nonvolatile ionic liquid, one in which the liquid electrolyte is made into a gel using an appropriate gelling agent to be quasi-solidified, and one using a solid semiconductor, such as a p-type semiconductor.
An ionic liquid is also called a room temperature molten salt, exists as a stable liquid in a broad temperature region, including in the vicinity of room temperature, and is a salt made from a cation and an anion. Since ionic liquids have very low vapor pressure and do not substantially vaporize at room temperature, they are not liable to volatilization or cause ignition, unlike typical organic solvents. Thus, they are proposed as a solution to a problem of decrease in cell characteristics caused by volatilization (see N. Papageorgiou et al., Journal of the Electrochemical Society (J. Electrochem. Soc.), United Kingdom, 1996, vol. 143 (10), p. 3099), for example.
In addition, when an electrolyte solution (liquid) is used as an electrolyte, the electrolyte solution may be exposed and leak (fluid leakage) during the manufacturing processes or upon breakage of the cell. In order to prevent fluid leakage, an attempt has been made to add an appropriate gelling agent (see Japanese Unexamined Patent Application, First Publication No. 2002-184478, for example).
However, ionic liquids suffer from a shortcoming in that the electrical resistance thereof is higher than that of an electrolyte solution obtained through dissolution into an organic solvent, such as acetonitrile.
In addition, gelling agents conventionally used for gel electrolytes include, for example, polyvinylidene fluoride, polyvinylidene fluoride—hexafluoropropylene copolymer, polyethylene oxide derivatives, polyacrylonitrile derivatives, amino acid derivatives. However, these gelling agents have a shortcoming in that the conversion efficiency of a photoelectric conversion element is significantly reduced compared to a case without adding such gelling agents since they are electrical insulators having very high electrical resistances.